LSD for a '73 911?

[TDK-UK]

Hello All...

I'm currently in the process of having my 911 restored by Alan.UK at Canford Classics here in the UK...



It's a RHD 1973 911T, but is having some considerable modifications, including S/RS spec brakes, suspension, oil cooler and increasing the capacity of the engine from 2.4 to 2.7 RS spec, consequently I'm going to have a LSD installed so that the car is more planted on the road and able to better utilise the increased power.

I've done a fair amount of research and have narrowed my choice down to two LSD's... an original low mileage ZF or a brand new Guard Transmission.

I'm going to quite some lengths with the bare-metal restoration and am trying to keep the car completely period, so my preference would probably be for the ZF LSD because if I use the GT LSD it'll be the only non-period non-Porsche part on the car (even though it'll be hidden inside the gearbox, unable to be seen). The cost of the GT LSD is also a bit of an issue as it's recently gone up in cost and the Pound isn't doing well against the Dollar, so it'll run about $3750 fitted - which isn't cheap!

There are some issues with using the ZF as it's not a brand new part (although very low mileage) and will need to be re-built, which isn't really a problem, but there's always the nagging doubt of a pre-used part. However, what is more of a problem is the need for the correct longer output flanges (61mm instead of the standard 57mm) for a coarse spline ZF LSD, which I don't currently have.

So a couple of questions...

1. Any advice or insight as to which might be my best option, ZF or GT?
2. Does anyone have any longer 61mm coarse spline output flanges that they'd be willing to part with for reasonable money?

Thanks...Simon.

[Chuck Miller]

Simon,

Get in touch with Jeff Erickson at AASE Motors
http://navcon.com/AASE.htm

He has a ZF/LSD dedicated rebuilding bench with all parts in stock.......
He did mine with great results.......

He just might have one for sale..... or know of one

Hope this helps,
Cheers

[TDK-UK]

Chuck,

Thanks for that, I'll drop Jeff a line and see if he can help as it's the longer coarse spline output flanges that I'm really after.

Simon.

[Tom Sch]

hello,

if you go the non period route - why not a http://www.quaife.co.uk/ LSD?
we made the best experiences with it

thanks

[turbochad]

I just went through this and decided to go with a GT TDB (torque bias diff) instead of a LSD. It is very similar to the Quaife, doesn't need to be rebuilt with every clutch replacement, lays down the power on throttle and is open under low torque (easy to park). If you drive the car on the street it is the way to go. The price is right too, around $1,600 for the unit. There are alot of junk ZF's out there and you can sink a lot of money into them when you may not need all the specific benefits of a LSD. Take a look at GT TBD or a Quaife. They area also good for track dubty, not as good under braking as a true LSD, but it is mere tenths of a second on lap times. Matt at GT is a great guy and will get you taken care of. He is currently running a 10% off spring sale.

The car looks great, enjoy!

[Flieger]

Do the worm-gear type torque-biasing differentials work under engine braking? I know they need to have torque applied to the ring gear (which is connected to the worms) in order to create the torque-biasing factor. But engine braking should create a torque just as acceleration does, just in the opposite direction. Would this cause the torque to transfer to the inside wheel, opposite of the acceleration bias direction?

[munich]

A good source of information: Limited slip. Steve knows what he is talking about. Cheers, Markus

[ttweed]

Do the worm-gear type torque-biasing differentials work under engine braking?

No. A TBD does not provide any differential locking on decel. It senses which wheel has better grip under acceleration only, and biases the power to that wheel automatically. This is where the name "Torsen" came from--"Torque-Sensing."

Under braking, a Torsen diff functions the same as an open diff. This is why they are actually preferred by autocrossers and scorned by big track racers. Locking the rear drive axles under braking makes for a more stable platform, but also induces some understeer tendency on turn-in while trail-braking. In autocross, with slow, tight corners that require rotating the car quickly, this can be a time-killer. On the big tracks, where corners are less sharp and much faster, understeer is not as much of an issue, and the added stability under high-speed braking of a clutch-type LSD is most appreciated.

YMMV,
TT

[Flieger]

Thanks. I thought that since there was still a torque, just in the opposite direction...

Now I understand thanks to you. The uni-directional locking is a result of the worm gear system? Cannot drive a screw with a gear but can drive a gear with a screw.

[Flieger]

I have heard that if the inside wheel is spinning, a TBD does not bias any torque. There needs to be some grip at both wheels. Is this true?